Loehle Proxy MD95-2011

One of Loehle’s proxies – the one that ends in the 16th century – has occasioned some interest and I took some time to investigate it. It is an alkenone SST estimate from ocean sediment core MD95-2011 from the Norwegian Sea (66 58.19N, 07 38.36E and 1048 m). The series ends in the 16th century because it was a piston core and the top of it was lost. However box core JM97-948/2A was drilled i the same location and provides a high resolution series right to the 1990s.

Both cores are in high-accumulation zones and have been closely sampled – a sample spacing of 0.5 cm in core JM97-948/2A and 1.0 cm in core MD95-2011 – yielding resolution of about 3.4 years and 8.4 years, respectively. Both have been discussed in a number of scientific reports and a number of relevant data sets have been archived at http://www.pangaea.de, a repository for much ocean sediment data.

The archive includes dO18 values for both N pachy dex and N pachy sin for both MD95 and JM97; the combined data being shown in the plot below, with a slight difference in color denoting the change in core. Do these measurements indicate that dO18 values in the Norwegian Sea are at unprecedented levels? You tell me.

While dO18 is sometimes used as a temperature proxy (e.g. Lonnie Thompson’s ice core), it has not been used as one here. SST was estimated in two different ways for this core – (1) by an analog based on foram distributions; (2) alkenone SST. The SST estimates from foram distribution are archived for both MD95 and Jm97 and are plotted below; for alkenone SST, only the MD95 values are archived. I’ve been informed that JM97 values have been calculated but they have not been archived or reported, apparently because of inconsistency problems. Here is a plot:

7 Comments

IIRC, error bars on SST estimates from foram frequency distributions are about one degree C. I don’t know what they are for alkenone-based estimates. It’s curious that the trends in the 8 kyr overlap are opposite.

I gather the update hasn’t been peer-reviewed published, and so won’t meet Craig’s Published Or Perish criterion. Could the authors be encouraged to published the update, perhaps as a short note in the same journal, so that it would qualify in future studies?

#3
The alkenones are produced by planktonic algae that grow in summer in the photic zone. This uppermost part of the ocean warms strongly in summer.
In this area, most of the forams are living in sub-surface water, perhaps 100-150 m deep. This is below the seasonal thermocline, and so does not warm in summer. The temperature at this depth is set during overturn in winter.
As the two proxies are sensitive to the temperature at different times of year, they can show different trends if summer and winter temperatures don’t have the same trend.

I do not have access to the full text of the Calvo et al. paper but they seem pretty confident on the method used, alkenones, showing a maximum 5.5-6.5 Kyrs BP. Any link to a free access to the full paper?. Thanks.

dO18 is sometimes used as a temperature proxy (e.g. Lonnie Thompsons ice core),

In Thompson’s 2003 paper (the infamous one whose Figure 7d, showing the MBH99 hockey stick
plus Jones 99 instrumental series, is the true source of AIT’s “Dr. Thompson’s
Thermometer”), and again in his 2006 PNAS paper, it is intimated that dO18 proxies temperature, but he does not actually calibrate any of his series to temp. Even his PNAS composite index is just a Z-mometer, not a true thermometer.
Has he actually calibrated any of his ice cores to temperature?